1. Field of the Invention
The present invention relates to an ion implantation apparatus, and more particularly to an ion implantation apparatus used for fabricating semiconductor devices such as a thin film transistor.
2. Description of the Prior Art
Among the processes for fabricating various semiconductor devices, ion implantation is indispensable and extremely important as a process for doping a semiconductor with impurities. A substrate on which semiconductor devices are formed has become large today since an image sensor, a liquid crystal display and the like have become large-sized and the productivity thereof has improved. As a result, an ion implantation apparatus for implanting ions on a large area is desired.
An ordinary ion implantation apparatus comprises an ion source, a mass spectrometer and an ion accelerator. Ions generated at the ion source are allowed to pass through the mass spectrometer to eliminate ions unnecessary to the ion implantation, and only the selected ions are accelerated by the ion accelerator. The accelerated ions are generally implanted into a substrate in a shape of an ion beam having a diameter of several mm. A dose of the ion implanted into the substrate can be determined by measuring an ion current flowing through the substrate.
When the ions are implanted into a large area substrate using the above described ion implantation apparatus, it is necessary to either scan the substrate mechanically or scan the ion beam electrically because the area of the substrate is large as compared with the diameter of the ion beam. This results in a problem that a larger substrate requires a longer time for the ion implantation. Moreover, providing a mechanical or an electrical scanning means causes another problem that the ion implantation apparatus becomes complicated, large-sized and expensive.
One of the techniques in which ions are easily implanted into a large substrate without the above-mentioned mechanical or electrical scanning means is disclosed in the Japanese Laid Open Patent Publication No. 63-194326. According to this prior art, ions generated by using a plasma discharge as the ion source are accelerated at a low voltage without allowing them to pass through the mass spectrometer and implanted into a substrate which has been heated to a predetermined temperature in a shower-like shape. Since the generated ions do not pass through the mass spectrometer, all kinds of generated ions including unnecessary ones are implanted. Furthermore, the dose of all the kinds of the ions is measured as the ion current. Therefore, in the prior art, the precise dose of a desired kind of ions can not be measured.
For example, in the ion implantation with the above ion implantation apparatus using PH.sub.3 gas diluted with H.sub.2, P.sup.+ and H.sup.+ are generated. Since only P.sup.+ makes the substrate conductive, the dose of P.sup.+ must be controlled. However, with this ion implantation apparatus, only the total dose of all the kinds of the ions including P.sup.+ and H.sup.+ can be measured. The dose of the desired kind of ions can only be experientially estimated based on the total dose. In such a case the control of the dose of a desired kind of ions is very inaccurate.